In 2010 at the Wuppertal Zoo in Germany, brain inflammation
killed one female polar bear, Jerka, and sickened her male
companion, Lars, whom veterinarians were able to save.

Brain-swelling disease, or encephalitis, can be caused by many
pathogens. But after investigating samples from Jerka, Lars and
nine other polar bears
researchers believe they have identified the culprit: a mishmash
virus that originated in zebras.

The bear-killing virus appears to have emerged when a portion of
equine herpes
virus transferred a portion of its genetic code — one known
for its role in causing disease that affects the nervous system —
into a second equine herpes virus, researchers say. (Equine
refers to members of the horse family, which includes zebras.)

This conclusion raises many questions. For starters, when and
where did this new virus emerge? And, is this
recombination, or mixing, what enabled the virus to jump
species and cause a fatal disease?

And, of course, how did the polar
bears at Wuppertal Zoo catch the virus?

The zebras are housed 223 feet (68 meters) from polar bears and
are not cared for by the same zookeepers. But the two parent
viruses have also been associated with fatal encephalitis in
other zoo species, such as gazelles and guinea pigs. The
researchers are now exploring whether or not wild mice and rats
could be carrying the virus.

"These
viruses do not seem to respect species boundaries and in
fact, we don't really know whether they have any," said study
researcher Klaus Osterrieder from the Freie Universität Berlin in
a statement.

Samples from a polar bear that had died years earlier at another
zoo tested positive for the same recombinant virus. This
indicates the virus had jumped between species more than once.

The research, led by Alex Greenwood, of Leibniz Institute for Zoo
and Wildlife Research Berlin, was published online today (Aug.
16) in the journal Current Biology.